The present invention relates to electronic filters and, more particularly, to an iris-less combline filter with improved performance characteristics and low manufacturing costs.
Conventional combline filters typically are used in front-end transmit/receive filters and diplexers of communication systems such as Personal Communication System (PCS), and Global System for Mobile communications (GSM). The combline filters are configured to pass only certain frequency bands of electromagnetic waves as needed by the communication systems. The combline filters can include uniform resonators rods or stepped resonator rods having steps.
FIG. 1 is a perspective view of a conventional combline filter 10 (with a cover removed therefrom) having uniform resonator rods. As shown, the combline filter 10 includes a plurality of uniform resonator rods 6 disposed within a metal housing 2, input and output terminals 12 and 14 disposed on the outer surface of the metal housing 2, and loops 16a and 16b for inductively coupling electromagnetic signals to and from the input and output terminals 12 and 14. The metal housing 2 is provided with a plurality of cavities 4 separated by dividing walls 4a. Certain dividing walls 4a have a well-known structure called a decoupling xe2x80x9cirisxe2x80x9d 8 having an opening 8a. The dividing wall 4a having the iris 8 is used to control the amount of coupling between two adjacent resonator rods 6, which controls the bandwidth of the filter. The resonator rods 6 resonate at particular frequencies to filter or selectively pass certain frequencies of signals inductively applied thereto. Particularly, input signals from the input terminal 12 of the combline filter 10 are inductively transmitted to the first resonator rod 6 through the loop 16a and are filtered through the resonance of the resonator rods 6. The filtered signals are then outputted at the output terminal 14 of the combline filter 10 through the loop 16b. 
A combline filter having stepped resonator rods is also known in the art. In such a filter, resonator rods having steps are used in lieu of the uniform resonator rods. The structure of this filter would be identical to that of the filter 10 shown in FIG. 1, except that the uniform resonator rods 6 are replaced with stepped resonator rods and different dimensions may be used. This type of filter also has the decoupling irises and multiple dividing walls to control the coupling coefficients between the stepped resonator rods.
In all these conventional combline filters, the passing frequency range of the filter is selectively varied by changing the lengths or dimensions of the resonator rods whether they be uniform rods or stepped rods. The operational bandwidth of the filter is selectively varied by changing the electromagnetic (EM) coupling coefficients between the resonator rods. The EM coupling coefficient represents the strength of EM coupling between two adjacent resonator rods and equals the difference between the magnetic coupling coefficient and the electric coupling coefficient between the two resonator rods. The magnetic coupling coefficient represents the magnetic coupling strength between the two resonator rods, whereas the electric coupling coefficient represents the electric coupling strength between the two resonator rods. Usually, the magnetic coupling coefficient is larger than the electric coupling coefficient.
To vary the EM coupling (i.e., EM coupling coefficient) between two resonator rods, the size of the iris opening disposed between the two resonator rods is varied. The larger the iris between the two resonator rods, the higher the EM coupling between the two resonator rods. This results in a wide bandwidth operation of the filter. In contrast, if the iris 8 has a smaller opening, a lower EM coupling between the resonator rods is effected, resulting in a narrow bandwidth operation of the filter.
Although effective, conventional combline filters with decoupling irises have a number of problems or drawbacks. For instance, the cavities, dividing walls and decoupling irises in the metal housing must be formed very precisely. Thus, the conventional combline filters require sophisticated milling, which increases costs and decreases throughput. Further, the plurality of dividing walls erected between the resonator rods of the filter significantly increases the signal loss known as xe2x80x9cinsertion lossxe2x80x9d. Moreover, if different bandwidth characteristics are desired for the combline filter, the metal housing of the filter must be re-machined to change the size of the iris openings. In this respect, the milling of the metal housing only allows the iris openings to be enlarged (e.g., by removing a portion of the dividing wall), but does not allow a reduction in the size of the iris openings. Thus, if a decrease in the coupling coefficient between the resonator rods is desired, the metal housing cannot be re-machined and the entire filter housing must be replaced to provide the desired coupling coefficient. Conventional combline filters are therefore restricted in applicability and adaptability.
Accordingly, there is a need for an improved combline filter which overcomes the above-described problems and other problems that are associated with conventional combline filters.
The present invention presents an innovative approach for controlling the EM coupling between resonators (resonator rods) which overcomes problems that are associated with conventional combline filters. Particularly, the present invention eliminates the use of decoupling irises and instead utilizes a capacitive coupling element to enhance electric coupling between resonators to control the overall EM coupling between the resonators. In one embodiment, the capacitive coupling element is a conductive rod supported by a non-conductive support member and disposed between two adjacent resonators. The capacitive coupling element is placed between the resonators, without contacting the resonators, where the electrical field is dominant, which improves the electric coupling between the resonators. In another embodiment, the capacitive coupling element is a conductive rod attached to one of two adjacent resonators, and is placed between the two resonators where the electrical field is dominant, which improves the electric coupling between the resonators. An increase in the electric coupling decreases the overall EM coupling between the resonators. Then, by selectively varying the dimensions of the capacitive coupling element which varies the amount of electric coupling present between the two resonators, the present invention controls the overall EM coupling between the two resonators without the use of decoupling irises. The use of capacitive coupling elements according to the present invention provides many advantages over conventional combline filters having decoupling irises. For example, a capacitive coupling element is more configurable than a decoupling iris. To modify the size of the iris openings to vary the EM coupling between the resonators, the entire metal housing needs to be re-machined. In contrast, in the present invention, only the capacitive coupling element needs to be reconfigured. Reconfiguration of the capacitive coupling element may involve trimming the ends of the capacitive coupling element, which can be easily accomplished, or replacing the capacitive coupling element with a new capacitive coupling element having different dimensions and/or configurations, which also can be easily accomplished. For instance, If less EM coupling is desired between two resonators, the existing capacitive coupling rod can be replaced with a longer capacitive coupling rod or a thicker capacitive coupling rod, or the height of the coupling rod can be increased. Thus, by merely varying the length, thickness, diameter, and/or height of the capacitive coupling elements and without requiring re-machining or replacement of the metal housing as in the conventional combline filters, the present invention permits easy modifications to EM coupling between the resonators.
Furthermore, the present invention eliminates the use of de-coupling irises and thereby reduces the number of dividing walls needed in the filter. This feature reduces the milling cost associated with manufacturing the filter, thereby greatly decreasing the manufacturing cost and time for the filter. This feature also reduces the insertion loss for the filter, which is typically caused by dividing walls, and thereby improves the performance characteristics of the filter. Moreover, the use of the capacitive coupling elements in conjunction with the resonators allows for signal attenuation zeros close to the passband of the filter, thereby providing high selectivity for the filter.
In one embodiment, the present invention is directed to a filter comprising a conductive housing, first and second resonators disposed in the housing, and at least one capacitive coupling element disposed between the first and second resonators, wherein there is no decoupling iris between the first and second resonators.
In another embodiment, the present invention is directed to a method of providing a filter, the method comprising the steps of providing a conductive housing, disposing first and second resonators in the housing, and disposing at least one capacitive coupling element between the first and second resonators, no decoupling iris existing between the first and second resonators.
In yet another embodiment, the present invention is directed to a method of providing a filter, comprising the steps of providing a conductive housing; disposing an integrated unit in the housing, the integrated unit including a resonator section and a capacitive coupling element section extending directly from the resonator section; and disposing a resonator in the housing a predetermined distance from the integrated unit, wherein there is no decoupling iris between the integrated unit and the resonator, and the capacitive coupling element section of the integrated unit controls coupling between the resonator and the resonator section of the integrated unit.